Si/SiC Ceramic Composite for Space Optical Mirror

Paper number

IAC-05-C2.3.03

Author

Dr. Yumin Zhang, Harbin Institute of Technology, China

Year

2005

Abstract

Silicon carbide offers some excellent characteristics including high stiffness, high toughness, low toxicity, low thermal distortion and potential cost and schedule advantages. These properties make silicon carbide very attractive for a variety of applications in precision optical structures, especially when considering space borne application. However, there are two difficulties to fabricate silicon carbide mirrors, namely how to gain large scale parts and how to form complex lightweight structure. 
In this paper, lightweight Si/SiC composite mirrors were prepared for the large pointer mirror or the primary mirror of the high resolution camera. The Si/SiC composite was a ceramic matrix of free silicon containing a bimodal distribution of silicon carbide grains. The green body of silicon carbide grains and free carbon was fabricated by slip casting. Inserts were used to form the complex lightweight structure. During silicon vapor infiltration process, silicon reacted with carbon to form new silicon carbide phase and the pores of green body disappeared. The resulting material was single phase alpha silicon carbide. Because there are very little shrinkage during drying and reaction process, the large scale production with complex shape has no crackles and distortions. The results indicated that reaction bonded silicon carbide can be used as space optical mirror substrates. Using electron beam physical vapor deposition, we prepared a silicon film on the mirror substrate. The properties and microstructure of Si/SiC composite and Si film were measured. 
We fabricated some space mirrors with various scales from 120 mm diameter to larger than 750mm ellipse long axis. A 120 mm hexagonal mirror with 15 mm thickness and 0.23 kg weight was polished and coated with Si film. Open back honeycomb lightweight structure was produced to gain 24.3 kg/m2 areal density. Polishing technique different from optical glass polishing was introduced to achieve less than 0.1 wavelength at 632.8 nm p-v surface error and less than 3 nm rms surface roughness. The stress and displacement under factual condition were estimated using finite element analysis method. 
Currently, experiments are under way to fabricate larger scale Si film and mirrors with larger than 1 m diameter.

Abstract document

IAC-05-C2.3.03.pdf

Manuscript document

IAC-05-C2.3.03.pdf (🔒 authorized access only).

To get the manuscript, please contact IAF Secretariat.