Ceramic matrix composites ar a new class of advanced materials intended for high temperature engineering applications. They exhibit enhanced strength and fracture toughness over monolithic ceramics due to the interaction of a propagating crack with the second phase reinforcing material. Theories for the mechanisms of toughening in brittle matrix composites, based on crack bridging and a fracture process zone, have been developed over the past few years. While these mechanisms have been studied in detail for both continuous fiber and whisker reinforced ceramics at room temperature, their fracture behaviors at elevated temperature are not well characterized. The first goal of this research is the development of a new experimental approach which will provide a quantitative measure of fracture processes at high temperature. The approach involves a simple experimental technique which permits simultaneous measurement of crack growth resistance curves and direct observation of microscopic fracture processes during controlled loading over a temperature range of 20-1,7500C. The second goal is to study the behaviors of three important and well characterized materials, representing a range of properties: SiC whisker reinforced alumina, Nicalon fiber/LAS glass ceramic, and carbon/carbon composite. The primary significance of the research is that it will provide the tools to determine the fundamental mechanisms of fracture and deformation of ceramics at high temperature. With these tools, materials developers can design ceramic matrix composites with improved fracture behavior and application designers will be able to predict the reliability of their structural components under high temperature conditions.
