Ceramic Matrix Composites

Abstract

Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and are easily damaged during fabrication and/or service. It is therefore understandable that an overriding consideration in ceramic matrix composites (CMCs) is to toughen the ceramics by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure. It is worth pointing out at the very outset that there are certain basic differences between CMCs and other composites. The general philosophy in nonceramic matrix composites is to have the fiber bear a greater proportion of the applied load. This load partitioning depends on the ratio of fiber and matrix elastic moduli, E _f/E _m. In nonceramic matrix composites, this ratio can be very high, while in CMCs, it is rather low and can be as low as unity; think of alumina fiber reinforced alumina matrix composite. Another distinctive point regarding CMCs is that because of limited matrix ductility and generally high fabrication temperature, thermal mismatch between components has a very important bearing on CMC performance. The problem of chemical compatibility between components in CMCs has ramifications similar to those in, say, MMCs. We first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs.

Problems

• 7.1. What are the sources of fiber degradation during processing of ceramic matrix composites?

• 7.2. Describe the advantages of using sol–gel and polymer pyrolysis techniques to process the ceramic matrix in CMCs.

• 7.3. Explain how a carbon fiber reinforced glass–ceramic composite can be obtained with an almost zero in-plane coefficient of thermal expansion.

• 7.4. Chemically, what is an alkoxide? Describe some of the alkoxides that can be used to obtain different ceramic matrixes in CMC.

• 7.5. Distinguish between interphase and interface.

• 7.6. Why is thermal shock resistance more of a problem in CMCs than in MMCs?