Thermo-Mechanical Properties of SiC-Mineral Binder Composites for Space Applications

Due to its excellent stiffness, thermal stability and low density, silicon carbide (SiC) is an excellent candidate for fabrication of lightweight substrates for space mirrors in telescopes and satellites. However, the strong Si-C covalent bond induces high thermal stability and mechanical strength which makes it difficult to manufacture dense SiC. Other ceramic mirror materials such as Cordierite (CO720) by Kyocera® and Spodumene (ZERODUR®) by Schott® are characterized by their light weight, near zero thermal expansion coefficient and excellent thermal properties. However, mirrors made of cordierite or spodumene have relatively low stiffness and unsatisfactory thermal conductivity. We hypothesize that composites made of SiC-Cordierite and SiC-Spodumene can serve as better mirror substrates characterized by high stiffness, high thermal conductivity and improved thermomechanical stability. The present study reports on the synthesis and characterization of SiC-Cordierite (SiC-Cord) and SiC-Spodumene (SiC-Spod) using powder metallurgy method. The densification and thermomechanical stability of the SiC-mineral composites are enhanced by a novel in situ mineralization mechanism at the interface between the SiC and mineral binders between 800 °C and 1200 °C. The densities of SiC-Cord and SiC-Spod composites were 2.74 g/cc and 2.61 g/cc, respectively, while the thermal conductivities were 6.737 W/m. K and 3.281 W/m. K, respectively. Polishing the SiC-Cord with SiC grit numbers 400–1200 and diamond/silica slurry resulted in a mirror surface with an average roughness of 2.32 nm on SiC particles. The nano indentation stiffness of the polished SiC-Cordierite composite measured 239.9 ± 20.6 GPa. The stiffness of the SiC-Cord composite is superior to that of pure cordierite (140 GPa) or Zerodur (80 GPa). The average Vickers hardness of SiC-Cordierite was 8.12 ± 4.5 GPa which was superior to that of Zerodur (6.08 GPa) and comparable to that of pure cordierite (8–8.5 GPa). The composite samples demonstrated high thermal shock resistance as indicated by their comparable compressive strength and dimensional stability before and after quenching from 1200 °C to room temperature in water. Taken altogether, the superior thermomechanical properties of SiC-Cordierite and SiC-Spodumene suggest their suitability for mirrors in space-based telescopes.