Large-scale fabrication of lightweight Si/SiC ceramic composite optical mirror

Abstract

Silicon carbide relative to other more traditional materials 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. In this paper, lightweight silicon carbide mirror was prepared for the large pointer mirror or the primary mirror of the high resolution camera. Reaction-bonded silicon carbide was used to fabricate the 250-mm diameter circular mirror with 30-mm thickness and 1.72-kg weight and the 120-mm hexagonal mirror with 15-mm thickness and 0.23-kg weight. Open back honeycomb lightweight structure was produced to gain 35.1 and 24.3 kg/m 2 areal density, respectively. Polishing technique different from optical glass polishing was introduced to achieve less than 0.3 λ at 632.8-nm p– v surface error and less than 5-nm rms surface roughness. The stress and displacement under factual condition were estimated using finite element analysis method. The reaction-bonded silicon carbide 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. The results indicated that reaction-bonded silicon carbide can be used as optical mirror substrates. Currently, experiments are under way to fabricate a large-scale lightweight Si/SiC optical mirrors with larger than 50-cm diameter.