Effect of surface condition on the bending strength of cordierite ceramics

Low expansion glass materials have been widely applied to space optics due to their extremely low thermal expansion properties and outstanding mirror polishing characteristics. Meanwhile, the authors have focused on cordierite ceramics as promising candidate materials for space optics because they have higher specific stiffness than the conventional low expansion glass materials, having extremly low coefficient of thermal expansion. In the previous study, we evaluated the material properties of cordierite ceramics and established the key technology for manufacturing lightweight structures and polishing precise mirror surfaces. However, a reliable strength evaluation on a large number of specimens with various surface conditions was insufficient. Because measurement of breakage stress is often susceptible to the method of evaluation, strength characteristics of the brittle material need to be evaluated using many specimens of different surface conditions and the same test standard and methodology. In this research, ring-on-ring test, an ISO standard (ISO EN 1288- 5), was selected as the strength test standard for brittle materials. The Weibull distribution was used as the extreme value distribution for the breakage analysis. The strength distribution of two cordierite ceramics (NEXCERA™ CD107 and CO720) with various surface conditions generated in grinding, polishing, and sintering processes were evaluated and compared with a low thermal expansion glass-ceramic (ZERODUR®). These results showed the cordierite ceramics tend to have higher strength and Weibull parameter than the glass-ceramic under some surface conditions. This paper describes the effect of the surface condition of cordierite ceramics on bending strength.