Ceramic Matrix Composites: Matrices and Processing

Abstract

Ceramic matrix composites (CMCs) generally consist of ceramic fibers or whiskers in a ceramic matrix. CMCs are designed to overcome the main drawback of monolithic ceramics, namely their brittleness. They are referred to as inverse composites, which is to say that the failure strain of the matrix is lower than the failure strain of the fibers, whereas it is the reverse in most polymer or metal matrix composites. Hence, under load it is the matrix which fails first. In order to prevent an early failure of the brittle fibers when the matrix starts to microcrack, the fiber/matrix (FM) bonding should be controlled during processing. CMCs are tough materials and display a high failure stress when the FM bonding is not too strong or too weak, which is usually achieved through the use of a fiber coating referred to as the interphase. The fabrication of CMCs requires specific processing techniques. Gas-or liquid-phase routes (or a combination of both), in which the interphase and the matrix are formed around the fibers from gaseous or liquid precursors, are usually preferred. CMCs are used as thermostructural materials under severe service conditions, for example, high temperatures under load and in corrosive atmospheres, such as combustion gases. The most commonly used CMCs are nonoxide CMCs, namely carbon/carbon (C/C), carbon/silicon carbide (C/SiC), and silicon carbide/silicon carbide (SiC/SiC), the fibers being specified first.