Chapter 8 - Environmental effects on high temperature mechanical behavior of ceramic matrix composites

Abstract

The focus of this chapter is to discuss the effects on high-temperature mechanical behavior of ceramic matrix composites (CMCs). This chapter will identify some of the mechanisms by which oxidation can occur in CMCs and delineate some of the factors that influence the oxidation reactions. There must first be transport of oxygen to the nonoxide phases for oxidation to occur in CMCs. Transport can occur either by gas-phase diffusion through porosity and microcracks or by solid-state diffusion through an oxide matrix phase. Often ,in the case of CMCs reinforced with monoxides such as SiC, the oxidation product can be solid, liquid, or a multiphase mixture of the two. Diffusion of oxygen must then occur through the oxidation product(s). Furthermore, the reaction product can subsequently react with the matrix, producing an additional phase. In all such cases involving a reaction product, the reaction rate of the composite can be different from that of either of the constituents. If the reaction products are gaseous, internal pressures develop that can be sufficient to nucleate cavities and/or initiate cracking. All of these processes can potentially have deleterious effects on the mechanical properties of the composite. These aspects are considered in detail in the subsections of the chapter. The concluding part of the chapter presents some issues that need to be addressed to move composites into engineering applications.