TABLE OF CONTENTS Monolithic ceramic materials contain many desirable properties, such as high moduli, high compression strengths, high temperature capability, high hardness and wear resistance, low thermal conductivity, and chemical inertness. As shown in Fig. 10.1, the high temperature capability of ceramics makes them very attractive materials for extremely high temperature environments. However, due to their very low fracture toughness, ceramics are limited in structural applications. While metals plastically deform due to the high mobility of dislocations (i.e., slip), ceramics do not exhibit plastic deformation at room temperature and are prone to catastrophic failure under mechanical or thermal loading. They have a very low tolerance to crack-like defects, which can result either during fabrication or in-service. Even a very small crack can quickly grow to critical size leading to sudden failure.
While reinforcements such as fibers, whiskers, or particles are used to strengthen polymer and metal matrix composites, reinforcements in ceramic matrix composites are used primarily to increase toughness. Some differences in polymer matrix and ceramic matrix composites are illustrated in Fig. 10.2. The toughness increases afforded by ceramic matrix composites are due to energy dissipating mechanisms, such as fiber-to-matrix debonding, crack deflection, fiber bridging, and fiber pull-out. A notional stress strain curve for a monolithic ceramic and a ceramic matrix composite is shown in Fig. 10.3. Since the area under the stress strain curve is often considered as an indication of toughness, the large increase in toughness for the ceramic matrix composite is evident. The mechanisms of...
