Compressive strength degradation in ZrB 2-based ultra-high temperature ceramic composites

Abstract

The high temperature compressive strength behavior of zirconium diboride (ZrB 2)–silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB 2–SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB 2–SiC–C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB 2–SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO 2 layer is formed, beneath which ZrB 2 oxidizes to ZrO 2. A significant reduction in room temperature strength occurs after 16–24 h of exposure to air at 1400 °C for the ZrB 2–SiC material, while for the ZrB 2–SiC–C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.