Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB2)-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 degrees 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 ZrB2-SiC material shows excellent strength of 3.1 +/- 0.2 GPa at room temperature and 0.9 +/- 0.1 GPa at 1400 degrees C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB2-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB2-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 SiO2 layer is formed, beneath which ZrB2 oxidizes to ZrO2. A significant reduction in room temperature strength occurs after 16-24h of exposure to air at 1400 degrees C for the ZrB2-SiC material, while for the ZrB2-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. (C) 2010 Elsevier Ltd. All rights reserved.